Grease Construction and Function

7/26/2019 Grease Construction and Function

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32 J U N E 2 0 0 8 T RI B O LO G Y & L U B R I C AT I O N T E C H N O L O G Y

he March Best Practices Notebook introduced

readers to important principles about the

nature of lubricants and lubricant raw materi-

als and how machines create an oil film when their

surfaces begin to interact. This month well exam-

ine the somewhat specialized area of grease con-

struction, performance attributes and issues of

thickener compatibility. These topics are a prelude

to lubricant selection, an issue well treat more in-

depth in a future article.

Grease composition, functionand performance

Sliding and rolling surface actions, coupled with

differences in speed, load, component size and

operating environment, all influence the type of oil

film that is expected to form and the viscometric

and additive properties that must be present with-

in the oil to protect machine components.

The oil itself must provide the lift and surface

protection functions in order for the grease lubri-

Articlehighlights:

Benefitsanddrawbacks

associatedwithgrease

usage.

Howgreasestif

fnessratings

areformulated.Whychemis

tryaffectskey

greaseperformancech

arac-

teristics.

Problemsassociatedwi

th

mixinggreases.

BEST PRACTICES NOTEBOOK

Understandinggrease constructionand function

Understandinggrease constructionand function

By Mike Johnson, CLS,CMRPContributing Editor T

A few key points about grease chemistry, ratingsand the specific properties needed for your

application.

32-2-38 bpnotebook 6-08 5/2/08 7:39 AM Page 32


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T R I B O LO G Y & L U B R I C A T I O N T E C H N O L O G Y J U N E 2 0 0 8 3 3

cant to be effective. All of the base oil and additive

type choices that were previously examined must

be revisited when selecting the grease. In addition

to those issues, the lubrication practitioner also

must select from a variety of thickener systems,

each with its own set of potential problems. Col-

lectively, grease selection seems like a simple task,

but if the reliability engineer wishes for the grease-

lubricated components to have life cycles resem-

bling oil-lubricated components, a significant

number of variables must be fully considered.

Greases, as a category, fall into a classification

of materials called non-Newtonian fluids. All fluids

that experience a change in viscosity with change

in shear stress are considered non-Newtonian.

Mayonnaise, ketchup and hair gel are examples of

common household non-Newtonian fluids. Greas-

es that exhibit shear-induced thinning are referred

to as thixotropic greases, and those that exhibit

shear-induced thickening are referred to asrheopectic greases.

Both thixotropic and rheopectic responses are

time dependent, meaning the thinning or thicken-

ing effect is more pronounced as the period of

shear stress increases. In other words, thixotropic

greases tend to liquefy as the elements in the

machine squeeze, push and otherwise stress the

fluid, and rheopectic greases tend to harden under

the same types of mechanical force.

It would be ideal for the grease to work thin

only at the point of immediate shear stress

(motion of the lubricated component) and then

instantaneously return to its original state themoment the stress force

stops. If this condition exist-

ed, the grease would liquefy

and function more like an oil

at the point of motion and

then reform and create an

isolating seal when the mo-

tion stops. Unfortunately, the

thinning and thickening ef-

fects tend to be permanent.

Greases perform their lu-

brication function over time

by gradually releasing oil in-to the working areas of the

contacting machine surfaces.

This function has been com-

pared to that of a sponge

gradually releasing its liquid

over a period of time. A more

practical image would be of a concentration of mil-

lions of microscopic sponges held inside the

machine, close to the working machine compo-

nents, each one gradually releasing oil into the

work zone. The greater the amount of sheer stress

that the grease experiences (likened to squeezing

the sponge) the faster the grease releases its hold

of oil. Of course, once the sponge has released the

oil, its usefulness is done. The oil and additive

types contained within the sponge, and eventually

released into the working areas of the machine, are

selected based on the type of frictional conditions

expected.

The permanent changes that the grease under-

goes are accelerated by rising temperatures,

increasing shear stress and mixture with other

greases. For these reasons the reliability engineer

must replenish the grease at an optimum time

cycle, with an optimum volume to arrive at a

replacement state that resembles the effectiveness

of an oil bath.

Grease: Pros and consPreviously we stated that machine lubricants have

to perform six key functions:

1. Separate surfaces

2. Minimize friction

3. Cool the machine part

4. Clean the working area

5. Prevent corrosion

6. Provide a means of hydro-mechanical energy

transfer.

With those being the stated objectives, there

are benefits and drawbacks associated with using

grease:

While it may be somewhat easier to identify

potential drawbacks for the use of grease vs. oil,

CONTINUED ON PAGE 34

Grease Advantages Grease Disadvantages

Reduced frequency of relubrication. Loss of component cooling.

Decreased cost of machine design for lubrication. Loss of component flushing.

Improved startup after a prolonged idle time. Localized heat spikes/hot spots.

Improved sealing effectiveness (seal assistance). Increased risk of lubricant incompatibility failure.

Reduced r isk of process contamination. Loss of contamination control functions (fi ltration).

More effective use of sol id f ilm additives. Increased r isk of lubricant oxidative fai lure.

Improved protection in high load/low speed machines. Machine component speed limits vs.Oil.

Increased risk of new lubricant contamination.

Storage stability limitations.

Increased risk of product variability/batch variability.Risk from relubrication practice:volume control.

Risk from relubrication practice:frequency control.

Risk from relubrication practice:viscosity selection.

Risk from relubrication practice:application failure.



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there clearly are advantages that make the use of

grease compelling.

Grease construction1

The word grease is derived from the Latin word

crassus, meaning fat. Early examples of systematic

lubrication with animal fats date back to circa 1,400

BC, with efforts to reduce friction on wheel axels.

From these very early roots, efforts to reduce fric-

tion were dependent on relatively abundant animal

and vegetable-based oils. Colonel William Drake

and his well-publicized oil well created a new way

to supply an arguably superior oil product, which

accelerated the move toward the use of mineral oil

and hastened the birth of the petroleum age.

Mineral oil, as it turns out, is a pretty good raw

material for lubricating surfaces. Taking the lead

from the (hand, hair, laundry) type soap manufac-

turing industry, which has been robust since the

early days of the period of the enlightenment, lubri-cant manufacturers adopted a soap manufacturing

technique called saponification to produce the

basis for building stable, useful petroleum greases.

Saponification is the chemical reaction that pro-

duces soap, which becomes the grease thickener.

High school chemistry teaches that a mixture of

an acid and a base produces a salt and water. If the

acid happens to be an organic or fatty acid, then

the product called is soap. Saponification occurs

following the mixing of a fatty acid with an alkali

component. The early fatty acids were produced by

cooking animal and vegetable fats with water to

separate glycerin and the inherent fatty acid. Theearly alkali component for soap manufacture was

derived from soda ash, which comes from the alka-

li remains of burned vegetable matter.

Owing to the benefit of many early chemical

industry developments, namely large-volume pro-

duction of organic acids (Stearic acid C17H35COOH

and Benzoic acid C6H5COOH are both common

organic fatty acids used in grease manufacture)

and metallic hydroxides (lithium, aluminum, calci-

um, sodium and barium hydroxides), grease manu-

facturers learned to create specialized soaps into

which mineral oils and additives were introduced

to deliver highly specialized product functions.In the manufacturing process the raw materials

for forming the soap, and some portion of the

lubricating oil itself, are combined in a mixing ves-

sel and blended/agitated to initiate the chemical

reaction between the selected metallic thickener

and fatty acid. The types and volumes of the mate-

rials used have a dramatic impact on the charac-

teristics of the finished products.

Simple soap greases (primarily lithium, alu-

minum and calcium) are created if only onea

long chainfatty acid is used during the soap for-

mation process. If the supplier wishes to prepare a

product to deliver better temperature resistance,

then he will add anothera short chainfatty acid

in an additional step. Complex soap greases (lithi-

um complex, aluminum complex, barium complex

and calcium sulfonate complex) are known for

superior temperature resistance vs. their simple

soap counterparts.

Once the soap has been formed (using only a

portion of the required lubricating oil) the balance

of the lubricating oil is added, along with the

remaining additives that are required to fortify the

product for optimum results. The grease is cooled,

milled to assure that the thickener is uniformly dis-

tributed through the other raw materials, and a

sample is removed for stiffness testing.

The grease stiffness rating is the primary differ-

entiating property that people use in selecting agrease. Grease stiffness is the characteristic that

enables the grease to either move freely or sit still

once placed into service. The higher the stiffness

rating the thicker the grease body. The National

Lubricating Grease Institute (NLGI) has devised a

nine-point scale that is used in conjunction with

ASTM D217 to grade grease stiffness. This method

provides the user with a stiffness rating ranging

between 000 and 6.

CONTINUED FROM PAGE 33

Figure 1. Grease penetration and consistency

(Source: The Lubrication Engineers Manual,Third Edition,(2007), The Association for Iron

and Steel Technology,Warrendale,Pa., p.92)



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In the testing process, the sample of grease is

cooled to 77 F, placed in a cup and smoothed over

and, as shown in Figure 1, a pointed cone is placed

on the surface of the grease. The operator releases

a fastener and allows the cup to sink into the

grease cup. Depending on the stiffness of the fin-

ished product, the cup may settle significantly or

only slightly. Once the cup comes to rest, the oper-

ator measures the degree of drop based on the dial

attached to the top of the rod supporting the cone.

Figure 2 represents the profiles established by

NLGI that dictate the grade or number of the fin-

ished product.

The thickness number or NLGI grade of the

grease only has implications relative to its require-

ment to either flow or not flow once applied to the

mechanical structure requiring lubrication. With

the exception of calcium sulfonate complex thick-

eners, the thickeners do not provide surface pro-

tection in addition to that provided by the base oil

and additives. Having a grease rated as an NLGI

No. 2 does not make it inherently superior or infe-

rior to an NLGI No. 1 grease.

Each soap type imparts different performance

properties worth noting. Reputable manufacturerswill work vigorously to overcome weaknesses and

enhance strengths of their respective products rel-

ative to the selected application types.

Key differences include: (See micrograph pictures).

Aluminum and aluminum complex greases

are known to have strong high-temperature

performance characteristics, including high


Figure 2. NLGI grease consistency values


Micrograph (A): Aluminum Complex GreasesUsing STRATCO Contactor Reactor

Micrograph (B): Calcium Greases UsingSTRATCO Contactor Reactor

Micrograph (C): Lithium Greases UsingSTRATCO Contactor Reactor

Micrograph (D): Lithium Complex Greases UsingSTRATCO Contactor Reactor



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dropping points and very good oxidation-

resistance. Aluminum-based greases also

tend to perform well in high-wash applica-

tions, resisting the force of process waters

and housekeeping wash hoses. These greas-

es tend to be stringy, and this characteristic

increases with rising sustained application

temperature. These greases have been known

to stiffen with extended use.

Calcium, calcium complex, calcium sulfonate

complex greases are best known for their

excellent wash- and water-resistance proper-

ties and can be fortified to also provide

strong corrosion-resistance properties. The

complex and sulfonate complex forms are

respected for high load-carrying capabilities

and have temperature limits on par with

other complex soaps. Calcium (hydrous and

anhydrous) are best used in low to moderatetemperature applications and have accept-

able stability at moderate temperatures.

Lithium and lithium complex greases are

very widely used. These have strong proper-

ties in a variety of categories. These greases

have excellent long-term work stability,

strong high-temperature characteristics and

have acceptable wash- and corrosion- resist-

ance capabilities. With additive enhance-

ment, the wash- and corrosion-resistance

can be improved. These also have good low

temperature shear performance, making

them suitable for extremely low temperatureapplications. The generally well-rounded per-

formance of these greases has made them

the product of choice for general purpose

grease relubrication in industrial and manu-

facturing environments.

There is another category of grease thickeners,

referred to generically as non-soap thickeners.

These thickeners are made with a variety of prod-

ucts and processes, and deliver a wide array of per-

formance results. The clay-based (bentonite) prod-

ucts and polyurea products represent the largest

market volumes of non-metallic thickeners.

There are appreciable differences in the manu-

facturing practices for non-soap greases. Bentonite

products are created by direct addition of the

thickener to the base and additive mixture. These

products require significant milling to assure uni-

formity. Polyurea thickeners are a type of polymer

formed by a reaction between amines and iso-

cyanates, which occurs during the grease forma-

tion process. Some of the common raw material

options are considered to be hazardous, requiring

careful environmental and health precautions for

their manufacture.

Common performance criteria for these non-

soap greases include:

Polyurea greases are preferred for use in ball

bearing applications, giving rise to their

broad-based acceptance in electric motor

applications. Polyurea greases contain little

to no heavy metals and have favorable high

temperature performance. Together these

two traits provide very good oxidation-resist-

ance. Polyureas tend to have fair work stabil-ity, wash- and corrosion-resistance. Some

polyureas have a low level of compatibility

with other soap and non-soap greases,

including other polyureas. Nonetheless,

there are individual products being manufac-

tured that demonstrate strengths in all of

these categories, including the important

issue of compatibility.

Bentonite greases were the original non-

melting grease. Bentonite is a type of clay.

The base oils tend to evaporate before the

clay material becomes hot enough to melt.

This is both a strength and weakness. Whenused for extended periods of time at elevated

temperatures, bentonite grease residues may

cause a filling of the housing that can make

long-term relubrication difficult. Bentonite

greases are incompatible with most other

grease types as well.

Figure 3 provides a breakdown of the general mar-

ket distribution of several greases by thickener type.2




Aluminumandalumin

um

complexgreasesare

knowntohavestron

g

hightemperaturep

er-

formancecharacteristics,

includinghighdroppi

ng

pointsandverygood

oxidation-resistance.



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In all grease products, the oil and the additives

have the primary roles of lifting, separating and

protecting surfaces, and the soap has the primary

role of holding the oil in reserve until it is needed

by the machine components. The March Best Prac-

tices Notebook suggested that lubricating oils canbe constructed with a wide variety of additive com-

pounds and base oil types and that some of those

raw materials may well compete with one another.

It was further suggested, since the lubrication prac-

titioner was not routinely privy to the ingredients

of any of the given products in use, that it would be

best to avoid mixing lubricant products. That state-

ment should be strongly reiterated, as it also per-

tains to the materials used as metallic thickeners

to form the grease products.

Many studies have been conducted over the

years that provide similar enough results that one

can take away from the discussion of grease com-

patibility a simple rule: Do not mix greases! Mixtures

of greases may fail to perform vs. either of the non-

mixed products in a variety of ways, including:

Not withstanding the changes that may occur

with incompatibilities with base oils and additives,practitioners could expect obvious and prompt

change in the greases stiffness (NLGI rating) and

loss of temperature resistance, with the other

changes occurring more gradually.

It is possible to know all of the ramifications

that may occur when two or more greases are

mixed, but not without a fair degree of testing and

analysis. Unless one is going to expend the finan-

cial resources and effort to effectively study the

Shear stability Increase or decrease in the firmness ofthe grease mixture.

Dropping point Decrease in the mixtures temperaturestability.

Oxidation-resistance Loss oxidation stability, increase inoxidation byproducts.

Wear-resistance Loss of AWand EP additive performance.

Rate of oil dissociation Premature loss of oil reservoir leading(bleed) to increased hardening.

Wash-resistance Loss of ability to withstand passive ordirect wash action of process solutions.


Figure 3. General market distribution of severalgreases by thickener type



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degree of compatibility between different types of

greases, one should avoid mixing if at all possible.

Figure 4 provides a useful general reference on

the issue of thickener compatibility that the practi-

tioner may use.3

Summary

Grease manufacturing is a highly complex and

detailed process. There are many different types of

materials that may be used, each of which has some

impact on the grease final performance characteris-

tics. The two broad categories of greases include

soap-thickened or non-soap thickened greases.

While the thickener type does clearly have an influ-

ence on the long-term behavior of the grease, the

majority of the surface protection work is provided

by the oil and the additive choices. The NLGI grease

stiffness rating system provides grease

manufacturers with a clear mechanism

for grading grease stiffness character-

istics.

Mike Johnson, CLS, CMRP, MLT, is the

principal consultant for Advanced Machine

Reliability Resources, headquartered in

Franklin, Tenn. You can reach him at

[email protected].

References

1. NLGI Lubricating Grease Guide, Fourth

Edition, (1996), National Lubricating

Grease Institute, Kansas City, Mo., pp.1.09-1.12.

2. Mang, T., and Dresel, W. (2007), Lubricants and

Lubrication, Second, Completely Revised and

Extended Edition, Wiley-VCH, p. 644.

3. Web reference: http://www.finalube.com/reference

_material/grease_compatibility_chart.htm.

TLT


Figure 4. Grease compatibility chart


Documents

Grease Construction and Function